1. Field of the Invention
The present invention relates to a laser module, and more particularly to a vertical cavity surface emitting laser (VCSEL) module used in optical communication fields.
2. Description of the Related Art
The development of a low-price optical module is a leading factor in a rapid growth in the development of FTTH (Fiber to the Home) network. In this spirit, a bidirectional optical module using a vertical cavity surface emitting laser (VCSEL) has been vigorously developed; however, it is not easy to form an optical module used to measure the optical output of a VCSEL at a low cost. Moreover, as the optical output direction of the VCSEL is perpendicular to the surface of VCSEL, it is difficult to reflect light emitted perpendicularly from the VCSEL and to induce the reflected light again toward a silicon optical bench (hereinafter, referred to as a “SiOB”), on which the VCSEL is mounted. Furthermore, in case that a plurality of devices including the VCSEL is integrated as a unit, it is difficult to control the reflection of light toward an undesired region.
Meanwhile, a Si-base PD is typically used as a monitor photodiode (MPD) for measuring the optical output of the VCSEL. Since, in a unidirectional (or transmitting or receiving) optical module, a VCSEL serving as a light emitting device and a PIN-PD (positive-intrinsic-negative PD) serving as a light receiving device are independently integrated, it is comparatively easy to integrate an MPD with the VCSEL. As such, the employment of a TO-can package proposes various alternatives for solving the above integration problem.
FIG. 1 is a schematic view of a conventional VCSEL module having a TO-can package structure. As shown, the conventional VCSEL module 100 comprises a substrate 110, a plurality of pins 112, an optical bench 120, a VCSEL 130, a housing 140, a lens 150, and an optical fiber 160.
A plurality of the pins 112 is used to apply the voltage to the substrate 110 and the optical bench 120 mounted on the pins 112. The VCSEL 130 is mounted at a central area of the upper surface of the optical bench 120, and MPD regions (not shown) for optical detection are formed at a peripheral area on the upper surface of the optical bench 120. A lower end of the housing 140 is attached to the substrate 110, and a hole having a circular shape is formed at an upper surface of the housing 140. The lens 150 is installed at the hole and serves to converge the light emitted upwards from the VCSEL 130 and to partially reflect the converged light downwards.
The optical fiber 160 includes a core 162 and a clad layer 164 surrounding the core 162, and is disposed above the housing 140 so that the optical fiber 160 is aligned with the lens 150. The light converged by the lens 150 is incident on the core 162 of the optical fiber 160, and the light reflected by the lens 150 is incident on the MPD regions of the optical bench 120. The MPD regions detect the light incident thereon as an electrical signal and monitor an output state of the VCSEL 130 from the above electrical signal.
When a bidirectional) module is manufactured, the VCSEL and the PIN-PD must be integrated into a single package. In this case, it is required to prevent the light emitted from the VCSEL from being reflected toward the PIN-PD. Therefore, monitoring of the output of the VCSEL using the reflected light is restricted. Also, in most of the bidirectional modules using a PLC (Planar Lightwave Circuit), since a gap between the VCSEL and the PIN-PD is excessively narrow, it is difficult to employ a method for detecting the reflected light. Moreover, in case that a plurality of devices are integrated into an array type, such as a parallel link, it is difficult to prevent light emitted from one device from being incident on the adjacent devices.
As described above, since the conventional VCSEL module monitors the output state of the VCSEL using reflected light, it is difficult to apply the conventional VCSEL to bidirectional optical modules.